Such cables are used in particular in the field of computing for high frequency data transmission, which may take place at about 100 MHz. Outwardly such cables are often of a flat shape. The bundles of insulated conductors may be in pairs, in triplets, or in quads. They may additionally be assembled together into one or more units. The insulated conductors of the bundles within a given unit are then advantageously assembled together at different pitches in order to build up the different bundles within the unit. Screening then surrounds each unit of the bundle.
In such a cable, each unit may be held together and screened by means of an insulating tape, e.g. made of polyester and having its outside face metallized. The tape is laid lengthwise or helically around the unit made up of bundles and it has overlapping margins. The unit may be made up of bundles having different assembly pitches so as to enable cross-talk between bundles to be reduced, thereby enabling a limit value of about -35 dB at 100 MHz to be achieved, but not any less.
In such a cable, the closeness of the screening to the insulating conductors that it surrounds also gives rise to relatively high values of linear capacitance and of linear attenuation for the cable.
Document WO-A-86/05311 describes a flat computer cable in which the individually insulated conductors are assembled together in pairs that are protected by individual screens, all of said insulated conductors being located in the same plane. In that cable, two insulated conductors that are positioned side by side are assembled together as a pair by a first insulating coating extruded around the two insulated conductors that are thus embedded therein. Each pair is protected by a metal screening tape surrounding it. Protected pairs that are disposed side by side are assembled together by means of a second insulating coating that is extruded around them, and that forms the overall outer protective sheath of the cable.
Advantageously, two opposite grooves are formed in the first coating between the two conductors of the pair concerned and in the second coating between the various pairs it assembles together, so as to facilitate the stripping that is necessary for gaining access to the conductors and for installing a terminal connector on the various pairs of the cable. For the same reasons, the first coating is made of polyvinyl chloride (PVC) that does not adhere to the insulation of the conductors which is made of polyethylene (PE) or of propylene (PP).
Implementing such a cable is relatively lengthy and very difficult. A large number of extrusion operations are required. In particular, after the first coating operation for assembling together the two conductors in each pair, and after the second coating operation for assembling pairs together, it requires accurate relative positions to be enforced between the various conductors in order to satisfy conditions initially defined for the desired electrical characteristics of the cable.
Stripping operations are also lengthy, by choosing a first coating of PVC on the PE or PP that insulates the conductors, the resulting dielectric properties are not as good as those which could have been obtained by using PE or PP or one of their copolymers, but in that case they would adhere very firmly to the insulated conductors which would then become difficult to strip.
In addition, by disposing the conductors in the same plane, the number of possible ways in which the conductors can be assembled together in the cable are limited since the conductors are assembled together in pairs only.